So, you have shelled out a week's pay for a really nice 'chalice coral' (Echinophyllia spp.) and you're wondering if it will keep its beautiful colors. This article can assist you in your quest and help you understand how lighting and water's physical qualities can affect your new coral.

This time, we'll examine those fluorescent proteins found mostly in those stony corals of suborder Faviina (although a number are from Families Lobophyllidae, Euphylliidae, and Agarciidae), with a few being isolated from soft corals and false corals. These stony genera include some of the highly colorful stony corals such as Echinophyllia, Trachyphyllia, Lobophyllia, Favia, Favites, Montastrea, Scolymia, and others. Soft corals include Dendronephthya,Clavularia, and Sarcophyton. The 'false coral' Ricordea are also known to contain this type of fluorescent protein. Altogether, almost 80 coral genera (at least 9 Families) are represented. Research suggests there are no nonfluorescent pigments (chromoproteins) in this group.

In this article, we'll compare information from various sources and make educated decisions on how to keep your Clade D corals colorful. I'll use the term 'pigment' from time to time - this is not technically correct, so please allow me some latitude.

Glossary

Absorption:

The process in which incident radiation is retained without reflection or transmission.

Clade:

For our purposes in this article, a grouping of pigments based on similar features inherited from a common ancestor. Pigments from corals includes Clades A, B, C, and D. Clades can refer to living organisms as well (clades of Symbiodinium - zooxanthellae - are a good example.)

Chromophore:

The colorful portion of a pigment molecule. In some cases, chromophore refers to a granular packet containing many pigment molecules.

Chromoprotein pigment:

A non-fluorescent but colorful pigment. These pigments appear colorful because they reflect light. For example, a chromoprotein with a maximum absorption at 580nm might appear purple because it preferentially reflects blue and red wavelengths.

Chromo-Red Pigment:

A newly described type of pigment possessing characteristics of both chromoproteins and Ds-Red fluorescent proteins. Peak fluorescence is at 609nm (super red).

Cyan Fluorescent Protein (CFP):

Blue-green pigments with fluorescent emissions in the range of ~477-500nm. Cyan and green pigments share a similar chromophore structure. Cyan pigments are expressed at lower light levels than green, red or non-fluorescent pigments.

Emission:

That light which is fluoresced by a fluorescent pigment.

Excitation:

That light absorbed by a fluorescent pigment. Some of the excitation light is fluoresced or emitted at a less energetic wavelength (color).

Fluorescence:

Absorption of radiation at one wavelength (or color) and emission at another wavelength (color). Absorption is also called excitation. Fluorescence ends very soon after the excitation source is removed (on the order of ~2-3 nanoseconds: Salih and Cox, 2006).

Green Fluorescent Protein (GFP):

Fluorescent pigments with emissions of 500-525nm.

'Hula Twist':

A bending of a fluorescent protein resulting in a change of apparent color. Molecular bonds are not broken; therefore the pigment can shift back and forth, with movements reminiscent of a hula dancer.

Kaede-type pigment:

A type of red fluorescent pigment with a characteristic primary emission at ~574-580nm and a secondary (shoulder) emission at ~630nm. Originally found in the stony coral Trachyphyllia geoffroyi, but common in corals of suborder Faviina and others.

Quantum Yield:

Amount of that energy absorbed and is fluoresced. If 100 photons are absorbed, and 50 are fluoresced, the quantum yield is 0.50.

Photobleaching:

Some pigments, such as Dronpa, loss fluorescence if exposed to strong light (in this case, initially appearing green and bleaching to a non-fluorescent state when exposed to blue-green light). Photobleaching can obviously cause drastic changes in apparent fluorescence. In cases where multiple pigments are involved, the loss of fluorescence (or energy transfer from a donor pigment to an acceptor pigment) could also result in dramatic shifts in apparent color.

Photoconversion:

A rearrangement of the chemical structure of a colorful protein by light. Depending upon the protein, photoconversion can increase or decrease fluorescence (in processes called photoactivation and photobleaching, respectively). Photoconversion can break proteins' molecular bonds (as with Kaede and Eos fluorescent pigments) resulting in an irreversible color shift, or the molecule can be 'twisted' by light energy (a 'hula twist') where coloration reversal are possible depending upon the quality or quantity of light available. This process is known as photoswitching).

Red Fluorescent Protein (RFP):

Those pigments with an emission of ~570nm and above. Includes Ds-Red, Kaede and Chromo-Red pigments.

Stokes Shift:

The difference in the maximum wavelength of fluorescent pigment excitation light and the maximum wavelength of the fluoresced light (emission). For example, a pigment with an excitation wavelength of 508nm and an emission wavelength of 535nm would have a Stokes Shift of 27nm.

Threshold or Coloration Threshold:

The point at which pigment production is sufficient to make its fluorescence visually apparent. The term threshold generally refers pigment production, although, in some cases, it could apply to a light level where a pigment disappears (as in the cases of photobleaching, or photoconversion).

Types of Fluorescent Proteins

There are at least 9 described types of coral fluorescent proteins. Note: Pigment types, such as green or red might not be structurally similar to another green or red pigment in a different clade. These include:

Cyan Fluorescent Proteins (CFP) - Cyan pigments are blue-green pigments with a maximum emission of up to ~500 nm. The chromophore structure of a cyan pigment is very similar to that of a green fluorescent pigment.

Green Fluorescent Proteins (GFP) - This group, by far, is the most numerous of the fluorescent proteins. The structure of green fluorescent chromophores is very similar to that of cyan fluorescent chromophores.

Yellow Fluorescent Proteins (YFP) - An unusual type of fluorescent protein with maximum emission in the yellow portion of the spectrum. Rare in its biological distribution, YFP is found in a zoanthid and some specimens of the stony coral Agaricia. Personally, I've noted yellow fluorescence in a very few stony corals (Porites specimens) here in Hawaii while on night dives using specialized equipment to observe such colorations (see www.nightsea.com for details on this equipment).

Orange Fluorescent Protein (OFP) - I've included this protein 'type' in an attempt to avoid confusion. OFP is used to describe a pigment found in stony coral Lobophyllia hemprichii and its name suggests a rather unique sort of protein. In fact, OFP is simply a variant of the Kaede-type fluorescent proteins.

Red Fluorescent Proteins (RFP) - A group of proteins including several different subtypes (Kaede, Ds-Red and Chromo-Red). Typically, fluorescent emission is in the range of ~580 nm to slightly over 600 nm.

Dronpa - A green fluorescent protein that loses fluorescent when exposed to blue-green light (~490 nm) but returns when irradiated with violet light at ~400 nm. Dronpa is a Clade D protein. Identified (so far) from a coral of Family Pectiniidae.

Chromo-Red Proteins - A new classification (Alieva et al., 2008) of a single fluorescent pigment found in the stony coral Echinophyllia. This chromo-red pigment has some qualities of a non-fluorescent chromoprotein, but fluoresces at a maximum of 609 nm.

Corals Containing Clade D Fluorescent Proteins

Figure 1. Coral taxonomy is in a state of constant flux, but this gives an idea of stony coral genera found in Suborder Faviina. Many of these animals contain Kaede or Clade D fluorescent proteins. Other coral genera contain them as well.

Figure 2. Corals other than those found in Suborder Faviina can contain Clade D fluorescent proteins.

Figure 3. Family Euphylliidae contains many coral species popular among hobbyists. Some contain fluorescent proteins.

Figure 4. Coral genera of Family Agariciidae.

These fluorescent proteins are known by two names - the first is 'Kaede-type proteins.' Kaede is Japanese for 'maple leaf' refering to these proteins' ability to transition from green to red color when irradiated with ultraviolet or strong blue light. These proteins are also known as 'Clade D proteins'. Many Kaede-type (or Clade D) fluorescent proteins are easily discernible by their spectral emissions - there is a distinctive secondary shoulder at ~630nm when the fluorescence is in the orange/red portion of the spectrum. See Figure 5.

Relationship of Clade D Proteins

Figure 6 demonstrates the relationship of proteins found in various coral genera. In most cases, the coral species is listed, followed the type of protein (for example, GFP for 'green fluorescent protein', further refined in some cases by the maximum fluorescent emission wavelength (e.g., 520). The scientific shorthand is also listed (e.g., Kaede, Dronpa, mc5, etc.) The listing is also color-coded for easy recognition of maximum fluorescence color.

Figure 6. This phylogenetic tree lists Clade D fluorescent proteins, and their relationship to one another. The coral's name is followed by 'type' of protein (CFP, GFP, and RFP) and maximum emission wavelength. In addition, some list the shorthand name used by researchers (such as rfloGFP found in Ricordea florida.

Clade D Corals

Now that we have the preliminaries out of the way, we can begin a detailed examination of information on Clade D fluorescent proteins. The following information is listed by coral genus in alphabetical order.

Agaricia

Common Name: Agaricia Order: Scleractinia Family: Agariciidae

Agariciacorals are found only in the Atlantic Ocean, and are unlikely to be found in home aquaria. Proteins found in these corals fluorescein the blue-green, green, yellow-green, and yellow portions of the spectrum.

Table 1. Agariciafluorescent proteins.

Pigment

Emission

2

3

Excitation

2

3

Found in:

P-486

486

*

*

426

*

*

Agaricia sp.

P-497

497

527

*

*

*

*

Agaricia sp.

P-503

503

*

*

*

*

*

Agaricia fragilis

P-508-620

508-620

*

*

*

*

*

Agaricia agaricites @ 60m

P-513

513

545

490

*

*

*

Agaricia sp.

P-515

515

*

*

*

*

*

Agaricia sp.

P-542

542

*

*

*

*

*

Agaricia undata @ 40m

P-557

557

600

545

*

*

*

Agaricia sp.

P-565

565

*

*

490

*

*

Agaricia humilis

Figure 7. This fluorescent protein fluoresces mostly in the blue-green portion of the spectrum.

Figure 8. This fluorescent protein from Agaricia glows a green color.

Catalaphyllia

Catalaphyllia specimens, commonly called Elegance or Wonder Corals, have been a mainstay in the hobby for years. The fleshy portion of the coral is usually fluorescent green, but tentacle tips can be orange. Some green fluorescent proteins in Catalaphyllia mature to red (the photoconvertible proteins described by researchers are likely found only in tentacles' tips).

Table 2.Catalaphyllia fluorescent proteins.

Pigment

Emission

2

3

Excitation

2

3

Found in:

P-517

517

*

*

509

*

*

Catalaphyllia jardinei

P-582

582

*

*

573

*

*

Catalaphyllia jardinei

Clavularia

Common Name: Clove Polyps Order: Alcyonacea Family: Clavulariidae

Only one fluorescent protein has been identified in Clavularia tissues, although there are obviously more. All soft corals' pigments are closely related.

Dendronephthya

It may come as a surprise to many hobbyists, but some Dendronephthya specimens contain zooxanthellae. It is uncertain if Dendronephthya specimens containing fluorescent pigments possess zooxanthellae as well. Green and orange/red fluorescent proteins have been identified in these soft corals, and some have the ability to change from green to red upon exposure to ultraviolet, violet, or blue light.

Table 4. Dendronephthya fluorescent proteins.

Pigment

Emission

2

3

Excitation

2

3

Found in:

P-508

508

*

*

494

*

*

Dendronephthya

P-575

575

*

*

*

*

*

Dendronephthya

P-575

575

*

*

557

*

*

Dendronephthya

Figure 10. A green fluorescent protein found in the often non-photosynthetic soft coral Dendronephthya.

Chalice corals are very popular, and for good reason. Their colors can be astounding and, not surprisingly, can be priced accordingly. Only a few fluorescent proteins are recognized in scientific literature, including 3 GFPs, one orange protein and a red one fluorescing at a maximum 0f 609nm. Interestingly, the P-582 protein found in Echinophyllia specimens is identical to another Kaede protein - the 'original' green-to-red transition protein found in Trachyphyllia geoffroyi. It is not unreasonable to assume that ultraviolet radiation and violet/blue light can cause the transition.

Coloration of the 'Watermelon chalice' can be explained by the green protein maturing to red. In areas of new growth, the coral would produce a GFP that would mature to red in older growth areas exposed to UV/violet/blue light. It is possible that the yellow 'eyes' in Figure 11 are due to an incomplete maturation where color mixing of green and red appear orange or yellow.

Table 5. Echinophyllia ('chalice' corals) fluorescent proteins.

Pigment

Emission

2

3

Excitation

2

3

Found in:

P-510

510

*

*

*

*

*

Echinophyllia echinata

P-520

520

*

*

*

*

*

Echinophyllia echinata

P-524

524

*

*

*

*

*

Echinophyllia echinata

P-582

582

*

*

*

*

*

Echinophyllia echinata

P-609

609

*

*

*

*

*

Echinophyllia

Figure 12. The beautiful green fluorescence found in a 'chalice coral.' Courtesy uniquecorals.com.

Favia

Common Name: Moon Coral Order: Scleractinia Family: Faviidae

Figure 13. Beautiful fluorescence of a Favia coral. Photo by the author taken at Barrier Reef Aquariums in Seattle, Washington.

Figure 14. This Favia appears to contain proteins fluorescing in the violet portion of the spectrum - but this might be due to color mixing. Courtesy uniquecorals.com.

A Favia coral probably holds the record for possessing the most fluorescent colors (over 20, although they were not individually identified). Exposure to ultraviolet, violet, or blue light can cause some of the green proteins to mature to red ones. When color mixing is considered, the number of possible glowing color becomes staggering.

Favites

Although science has recognized only blue-green and green fluorescence in Favites corals, aquarists know that orange/red fluorescent proteins are found in these corals as well.

Table 7. Favites fluorescent proteins.

Pigment

Emission

2

3

Excitation

2

3

Found in:

P-478

478

*

*

*

*

*

Favites heliocora

P-498

498

*

*

*

*

*

Favites heliocora

P-500

500

*

*

*

*

*

Favites heliocora

P-520

520

*

*

*

*

*

Favites abdita

Figure 19. Excitation and emission wavelengths become blurred when fluorescence can be absorbed by another pigment. This Favites probably appears green.

Galaxea

Common Name: Galaxy coral Order: Scleractinia Family: Euphyllidae

The highly aggressive coral Galaxea is known to contain several green fluorescent proteins.

Table 8. This listing of Galaxea proteins may seem redundant. It is actually compiled from several references and is complete at the time of this writing.

Pigment

Emission

2

3

Excitation

2

3

Found in:

P-505

505

*

*

492

*

*

Galaxea fascicularis

P-505

505

*

*

*

*

*

Galaxea fascicularis

P-506

506

*

*

*

*

*

Galaxea fascicularis

Figure 20. Excitation and emission of the GFP found in the aggressive stony coral Galaxea.

Lobophyllia

Common Name: Meat Coral Order: Scleractinia Family: Lobophyllidae

Some Lobophyllia hemprichii specimens contain the fluorescent protein known as 'Eos' (the goddess of dawn in Greek mythology). Eos is photo-activatible, and irreversibly transitions from green (emission at 516nm) to orange/red (581nm) when exposed to ultraviolet/violet light.

Figure 21. The 'meat coral' Lobophyllia. Courtesy uniquecorals.com.

Figure 22. An interesting mix of green and orange fluorescence in a Lobophyllia specimen. Photo by the author.

Table 9.

Pigment

Emission

2

3

Excitation

2

3

Found in:

P-483

483

*

*

572

454

510

Lobophyllia hemprichii (red)

P-515

515

*

*

*

*

*

Lobophyllia hemprichii (red)

P-516

516

*

*

*

*

*

Lobophyllia hemprichii

P-517

517

Lobophyllia hemprichii

P-519

519

*

*

*

*

*

Lobophyllia hemprichii (red)

P-574

574

543

Lobophyllia hemprichii (red)

P-580

580

*

*

*

*

*

Lobophyllia hemprichii (red)

P-581

581

*

*

*

*

*

Lobophyllia hemprichii (red)

Figure 23. A green fluorescent protein found in the 'meat coral' Lobophyllia.

Figure 24. A GFP found in Lobophyllia hemprichii (at a pH of 7.0).

Figure 25. The orange/red fluorescence of P-581 shows the distinctive shoulder of Kaede proteins at ~630nm.

Montastraea

A recent revision to coral taxonomy has renamed Montastraeafaveolata as Orbicella faveolata. Whatever the coral genus, the fluorescent proteins found in Montastraea species appear to be closely related.

Figure 26. A red Montastraea specimen. Photo courtesy of Jake Adams.

Although corals of the genus Montastraea are found in the Atlantic and Pacific, only Atlantic species have been examined for fluorescent proteins. It is not unreasonable, in my opinion, to expect Pacific Montastraea species to contain similar, if not identical, fluorescent proteins.

Mycedium

At least one coral in Family Pectiniidae contains the fluorescent protein known as Dronpa (a disappearing Ninja according to folklore). This green fluorescent protein photo-bleaches when exposed to blue-green light (~490nm). Upon exposure to violet light (~400nm), the green fluorescence returns. Since many lamps generate both wavelengths, it is uncertain how broad spectrum light affects coloration in aquarium corals.

I find it ironic that a system based on order is in a state of constant chaos, where coral families are re-sorted according to the latest information. In any case, if Plesiastrea's Scleractinian Family is in doubt, it seems certain that the fluorescent protein found in these corals falls into Clade D.

Quite a few fluorescent proteins have been identified in Plesiastrea corals including CFP's, GFP's, yellow-green proteins, in addition to orange and super-reds.

Table 13. A listing of fluorescent proteins found in Great Barrier Reef Plesiastrea specimens.

Pigment

Emission

2

3

Excitation

2

3

Found in:

P-472

472

*

*

*

*

*

Plesiastrea verispora (blue morph)

P-473

473

*

*

*

*

*

Plesiastrea verispora (blue morph)

P-473

473

*

*

*

*

*

Plesiastrea verispora (green morph)

P-477

477

*

*

*

*

*

Plesiastrea verispora (blue)

P-479

479

*

*

*

*

*

Plesiastrea verispora (blue morph)

P-479

479

*

*

*

*

*

Plesiastrea verispora (green morph)

P-482

482

*

*

*

*

*

Plesiastrea verispora (blue)

P-485

485

*

*

*

*

*

Plesiastrea verispora (blue morph)

P-485

485

*

*

*

*

*

Plesiastrea verispora (green morph)

P-488

488

*

*

*

*

*

Plesiastrea verispora (blue morph)

P-489

489

*

*

*

*

*

Plesiastrea verispora (blue morph)

P-489

489

*

*

*

*

*

Plesiastrea verispora (green morph)

P-492

492

*

*

*

*

*

Plesiastrea verispora (blue)

P-495

495

*

*

*

*

*

Plesiastrea verispora (blue morph)

P-495

495

*

*

*

*

*

Plesiastrea verispora (green morph)

P-497

497

*

*

*

*

*

Plesiastrea verispora (blue morph)

P-497

497

*

*

*

*

*

Plesiastrea verispora (green morph)

P-501

501

*

*

*

*

*

Plesiastrea verispora (blue morph)

P-503

503

*

*

*

*

*

Plesiastrea verispora (green morph)

P-505

505

*

*

492

*

*

Plesiastrea verispora

P-508

508

*

*

*

*

*

Plesiastrea verispora (green morph)

P-511

511

*

*

*

*

*

Plesiastrea verispora (green morph)

P-512

512

*

*

*

*

*

Plesiastrea verispora (blue morph)

P-512

512

*

*

*

*

*

Plesiastrea verispora (green morph)

P-515

515

*

*

*

*

*

Plesiastrea verispora (green morph)

P-515

515

*

*

*

*

*

Plesiastrea verispora (green morph)

P-518

518

*

*

*

*

*

Plesiastrea verispora (green morph)

P-540

540

*

*

*

*

*

Plesiastrea verispora (blue morph)

P-540

540

*

*

*

*

*

Plesiastrea verispora (green morph)

P-574

574

550

*

*

*

*

Plesiastrea verispora

P-580

580

*

*

*

*

*

Plesiastrea verispora (blue morph)

P-580

580

*

*

*

*

*

Plesiastrea verispora (green morph)

P-620

620

*

*

*

*

*

Plesiastrea verispora (blue morph)

P-620

620

*

*

*

*

*

Plesiastrea verispora (green morph)

Figure 34. Blue-green light is fluoresced as green light in this Plesiastrea.

Figure 35.Green-yellow light is absorbed by this Plesiastrea, and is most strongly fluoresced in the orange portion of the spectrum.

Ricordea

Ricordeaspecies have a number of devotees and for good reasons - their ease of maintenance and beautiful colorations make them excellent candidates for reef aquaria. Scientists have shown aquarists that fluorescent proteins within these corals can transition from green to orange/red. It is supposed that blue light causes this transformation.

Figure 36. A gorgeous Ricordea specimen. Courtesy Reef Wholesale.

Table 14. Ricordea fluorescent proteins.

Pigment

Emission

2

3

Excitation

2

3

Found in:

P-510

510

*

*

*

*

*

Ricordea florida

P-513

513

*

*

*

*

*

Ricordea florida

P-515

515

*

*

*

*

*

Ricordea sp.

P-517

517

574

*

506

566

*

Ricordea florida

P-517

517

*

*

506

*

*

Ricordea florida

P-518

518

*

*

508

475

*

Ricordea florida

P-518

518

*

*

*

*

*

Ricordea florida

P-518

518

*

*

*

*

*

Ricordea florida

P-520

520

*

*

*

*

*

Ricordea florida

P-573

573

510

*

*

*

*

Ricordea florida

P-574

574

517

*

506

566

*

Ricordea florida

P-576

576

*

*

*

*

*

Ricordea florida

P-587-590

587-590

*

*

*

*

*

Ricordea florida (mouth)

Sarcophyton

Sarcophytoncorals have been a mainstay of the hobby for many years. They are easy to maintain in captivity and, in good water flow, offer a dynamic sense to an aquarium otherwise full of rigid, stony corals. In addition, their green fluorescence adds beauty to any reef aquarium.

Figure 37. The soft coral Sarcophyton. Courtesy uniquecorals.com.

Table 15. Only one GFP is officially recognized as being from the soft coral Sarcophyton.

Pigment

Emission

2

3

Excitation

2

3

Found in:

P-500

500

*

*

*

*

*

Sarcophyton sp.

Scolymia

Colorful Scolymia specimens are popular - and often expensive. Their fluorescent proteins range in coloration from blue-green to orange-red. Some consider specimens in this genus to be a bit touchy in captivity if they are not fed regularly.

Trachyphyllia

The Family Trachyphylliidae is composed of one genus - Trachyphyllia. It was from this coral that the Kaede group of fluorescent proteins was originally described. 'Kaede' is Japanese for 'maple leaf', referring to its green to red transition when exposed to ultraviolet, violet, or blue light. Note that the intermediate transition from green to red can produce yellow or orange intermediates.

Table 17. The green fluorescent protein (FP-518) transitions to a protein fluorescing red (at 582nm) upon exposure to ultraviolet, violet, or blue light. A number of intermediate colors (such as yellow) are possible due to color mixing.

Pigment

Emission

2

3

Excitation

2

3

Found in:

P-518

518

*

*

*

*

*

Trachyphyllia geoffroyi

P-582

582

*

*

558

*

*

Trachyphyllia geoffroyi

Figure 44. This protein from Trachyphyllia begins as a green fluorescent protein but changes to reddish-orange under ultraviolet radiation or violet/blue light.

Figure 45. Absorption spectra of the Kaede protein transitioning from green to red.

Fluorescent Proteins Likely to Belong to Clade D, but Unconfirmed

Based strictly on genera, these corals probably contain Clade D fluorescent proteins and may act similarly to others in the group.

Genus: Colpophyllia Family: Mussidae

Table 18. A Colpophyllia fluorescent protein.

Emission

2

3

Excitation

2

3

Found in:

515

*

*

*

*

*

Colpophyllia natans

Genus: Diploria Family: Mussidae

Table 19. Diploria fluorescent proteins.

Emission

2

3

Excitation

2

3

Found in:

P-486

486

*

~448

*

*

Diploria labyrinthiformis

P-517

517

575

*

*

*

Diploria strigosa

Genus: Hydnophora Family: Merulinidae

Table 20. A Hydnophora fluorescent protein.

Emission

2

3

Excitation

2

3

Found in:

P-492

492

*

*

443

*

Hydnophora grandis

Genus: Manicina Family: Mussidae

Table 21. A Manicina fluorescent protein.

Emission

2

3

Excitation

2

3

Found in:

P-487

487

515

475

*

*

Manicina areolata

Genus: Mycetophyllia Family: Mussidae

Table 22. Mycetophyllia fluorescent proteins.

Emission

2

3

Excitation

2

3

Found in:

P-515

515

*

*

*

*

Mycetophyllia lamarckiana

P-515

515

*

*

~494

*

Mycetophyllia sp.

Genus: Leptoseris Family: Agariciidae

Mikhail Matz (one of the world's leading experts on coral fluorescence) argues that this protein was isolated from this deep-water specimen through not use of water but a polar solvent instead. This fact suggests the protein may be different from all others.

Table 23. The mysterious fluorescent protein found in Leptoseris.

Emission

2

3

Excitation

2

3

Found in:

P446(?)

446

*

*

380

*

Leptoseris fragilis

Genus: Pavona Family: Agariciidae

The only fluorescent protein identified so far from Pavona fluoresces in the green portion of the spectrum. However, some Pavona specimens are fluorescent orange. Is this an example of photoconversion from green to orange?

Figure 46. A Hawaiian Pavona specimen. Photo by the author.

Figure 47. The GFP found in Pavona varians.

Effects of pH

pH is known to affect the absorption and fluorescent characteristics of some proteins. These changes tend to be slight, but it is not unreasonable to believe (supported by anecdotal evidence) that large swings in apparent color could be caused by extreme pH modulations (caused by an overdose of carbon dioxide or kalkwasser). See Figures 48 and 49.

Figure 48. The 'normal' characteristics of P-516.

Figure 49. Compare the characteristics of this P-516 exposed to low pH (7.0) to that in Figure 48.

Figure 50. Note that yellow-green light excites the red fluorescence in this Lobophyllia coral. Note also the low pH - this affects the absorption and emission of the protein.

Discussion

Proteins of Clade D are found in over 80 coral genera (at least 9 families). Evidence suggests many are photo-convertible by ultraviolet radiation and violet/blue light (although blue LEDs - producing practically no ultraviolet radiation and little violet light) are known to induce expression and maintain coloration in many corals.)

Table 24. Conversion of green fluorescent proteins to the Kaede type orange/red pigments has been observed when the animal is exposed to Ultraviolet-A radiation (UV-A), and violet/blue wavelengths. Care should be taken when experimenting with dosages of UV-A in attempts to promote coloration. Most plastic 'splash guards' are transparent to at least some UV-A wavelengths and it is usually not necessary to deliberately increase UV-A levels in order to induce production of pigments. With this said, there is some anecdotal information (based on my personal observations) that color of at least some pigments might increase in at least some coral species when UV radiation is slightly increased.

From

To

Host

Clade/Pigment

Activator

P-505

508/572

Montastraea cavernosa

D

? - But most likely blue light

P-505

506/566

Ricordea florida

D

? - But most likely blue light

P-508

575

Dendronephthya sp.

D

Blue Light @ 488nm

P-508

575

Dendronephthya sp.

D

UV-A at 366nm

~516

582

Montastraea cavernosa

D

Depth- light -related?

P-516

581

Lobophyllia hemprichii

D

UV @ 390nm/Violet Light ~400nm

P-517

574

Ricordea florida

D

UV/Violet Light

P-517

580

Montastraea annularis

D

UV/Violet Light

P-517

593

Favia favus

D

UV & Violet (350-420nm)

P-518

582

Trachyphyllia geoffroyi

D

UV - Violet Light (350-410nm)

P-519

580

Montastraea cavernosa

D

UV - Violet Light

However, many blue-green and green fluorescent proteins are present in conditions of low light, suggesting their presence is not related to light intensity (or perhaps generated in conditions of very low light).

To confound matters, at least one colorful protein (Dronpa) is known to photo-bleach when exposed to blue-green light at 490nm, but the fluorescence returns if exposed to blue light. Fortunately (for aquarists), researchers have found that Dronpa is not closely related to many other Clade D proteins (see Figure 6).

pH

Schreiner et al. (2011) report the red fluorescence of the Eos protein (from Lobophyllia sp.) decreases when exposed to an acidic pH (<7.0), while the green fluorescent emission is unaffected (this in contrast to others' observations). Apparently, some GFPs are affected by pH while others are not. When looking at the fluorescent protein phylogenetic tree, we see the red Eos protein is clustered with several other red fluorescent proteins, including those from Trachyphyllia, Catalaphyllia, Mycedium, Echinophyllia, and Scolymia. We might expect the red fluorescence of these pigments to be affected similarly by low pH.

An acidic pH would be an unusual condition within a reef aquarium, but it is possible, perhaps as a result of an overdose of carbon dioxide through use of a calcium reactor or similar device. On the other hand, a high pH (caused by an overdose of kalkwasser) might affect apparent coloration as well.

To summarize, light (particularly violet and/or blue) seems to be the environmental trigger for inducing the production of fluorescent proteins (ultraviolet light can, in many cases, also cause their production. However, the use of LEDs and their almost total lack of UV production strongly suggest UV is not necessary.) Another factor - pH - is known to slightly alter the fluorescent emission of these 'pigments'.

Many Clade D proteins' fluorescent emissions change as the protein ages (assuming the exposure to violet/blue light continues. This is seen dramatically in the 'watermelon chalice' as the green margin edges matures and turns red). The transition from green to red is responsible for an almost unlimited number of intermediate colors. However, recall that many Clade D proteins do not make a transition from green to red.

Many of the Clade D proteins cloned for use in biomedical research are stable at higher (human body temperatures) suggesting they are relatively unaffected by temperature modulations.

Our understanding of coral fluorescent proteins is due largely to experiments conducted in the biomedical fields and we have come a long way since corals' colors were attributed to photopigments such as peridinin (erroneously thought at one time to lend a brick-red color to corals). However, early observations that corals tended up to 'color up' when exposed to blue light have been proven to be essentially correct. With the recent revelation that is possible to provide at least as much (or in some circumstances, more) blue light than corals could expect to 'see' in nature, further experiments can proceed in examining the effects of altered spectra on coral photosynthesis and expression of coloration (see here for details: http://www.advancedaquarist.com/2013/11/aafeature ).

Acknowledgements

Taxa are classified according to World Register of Marine Species (WoRMS) www.marinespecies.org and are correct as of early October 2013. Any errors are mine.

Many thanks to Joe Caparatta at Unique Corals (www.uniquecorals.com) for supplying many of the photographs.